1. Field of the Invention
The present invention relates to a permanent-magnetic type rotary electric machine, and for example it relates to a motor used in an electrical power steering device for a vehicle.
2. Description of the Related Art
FIG. 20 is a schematic diagram showing a general electrical power steering device for a vehicle. In FIG. 20, the electrical power steering device is provided with a column shaft 31 for transmitting steering force from a steering wheel 30. A worm gear 32 is connected to the column shaft 31, and it transmits the output (torque, rotational speed) of a permanent-magnetic motor 34 driven by a controller 33 while changing the rotational direction to the vertical direction and decelerates the motor at the same time to increase the assist torque. Reference numeral 35 represents a handle joint, and it can transmit the steering force and also change the rotational direction. Reference numeral 36 represents a steering gear. The steering gear 36 decelerates the rotation of the column shaft 31 and simultaneously converts the rotation concerned to the linear motion of a rack 37 to obtain a required displacement. The linear motion of the rack 37 moves wheels (not shown), thereby enabling turnabout (change of travel direction) of a vehicle, etc.
In the electrical power steering device as described above, pulsation of torque occurring in the permanent-magnetic motor 34 is transmitted to the steering wheel 30 through the worm gear 32 and the column shaft 31. Accordingly, when the motor 34 generates large torque pulsation, it is impossible to obtain a smooth steering feeling.
The torque pulsation means a phenomenon that the torque is not set to a fixed value due to the effect of magnetomotive force harmonics at a rotor side and the torque varies in accordance with the angle of the rotor under the state that current is made to flow through an armature winding. For example, it is unevenness of torque occurring even when the waveform of current flowing through the armature winding is a sine wave.
In order to reduce such torque pulsation, it has been hitherto proposed to subject the rotor to skewing. For example, there has been proposed a method of preventing gradual decrease of a torque ripple constant which is caused by increase of a skew angle (for example, see JP-A-8-126279), or a method of specifying a skew angle to reduce the unevenness of torque under the state that current is made to flow through an armature coil (for example, see JP-A-11-69679).
In the conventional permanent-magnetic type rotary electric machine, the skew angle has not been set in consideration of both of amplitude and phase of the fifth and seventh higher harmonic waves of the no-load induced voltage, and thus the effect of the magnetomotive force higher harmonics at the rotor side cannot be sufficiently suppressed.